The present invention refers to the modulation and demodulation method, as well as the transmitter and receiver which makes it possible to transmit and receive data by means of any transmission means, particularly when it is necessary or preferable to use spread spectrum techniques.
The spread spectrum concept was developed for use in military communications due to its immunity features towards noise and interference. Its principle is based on the use of certain binary sequences having certain features similar to noise and which, however, a receiver which knows said sequence is capable to detect as a signal. In the same manner, the compression of impulses by means of binary sequences is useful also in RADAR, SONAR, and echography applications since it allows to improve the resolution of the detected objects. However, in recent years its use has become widespread in space applications and civil communications such as mobile telephony, DS-CDMA (Direct Sequence Code-Division Multiple Access), radio telephone access loops, Internet access, wireless local area networks, deep space communications, etc. all of which are based on digital modulation by means of using sequences which are suitable for this type of applications due to their autocorrelation and cross-relation features. Therefore international organizations (IEEE, UIT, etc.), have begun normalizing and standardizing modulation systems which facilitates the use of certain sequences to modulate the transmitted binary data and thus obtain characteristics which makes it possible to use, among others, certain frequencies reserved for industrial, scientific and medical applications (ISM bands) and whose use and exploitation do not require any kind of administrative license. The need to send as much information as possible with the same bandwidth has made the telecommunications industry to develop commercial applications which use the IEEE 802.11 standard for the transmission of information by radio in local networks obtaining increasingly higher speeds by means of the use of binary sequences such as the 11-bit Barker (to obtain a minimum processing gain of 10.4 dB) or 8-bit Walsh, and different modulation techniques (BPSK, QPSK, MBOK, QMBOK, etc.) which makes it possible to attain transmission speeds of up to 11 Mbps. This standard makes it possible to work within three frequency bands with a null-to-null bandwidth of 22 MHz, in the so-called 2.4 GHz band.
Likewise, reliable transmission methods are needed for the so-called deep space communications between spaceships and the bases on Earth, allowing a big processing gain due to the need to limit the emission power of the ship's transmission equipment, and due to the reduced signal to noise ratio of said signals when they are received.
In the present applications (FIG. 1) the length of the coding sequence (Barker, PN, Walsh, etc.) determines both the processing gain and the bandwidth used. Generally, the transmission speed will be reduced if we attempt to increase the processing gain, which is why a compromise between the two parameters must always be found. The transmission speed may be increased by increasing the number of modulation phases, however, the restrictions of this technique increase with the decrease in the signal to noise ratio during reception.
Based on the above it can be deduced that there is a need for a spread spectrum digital modulation technique which on the one hand makes it possible to increase transmission speed and on the other to obtain a bigger processing gain to make it possible to reduce the needed transmission power or improve the signal to noise ratio during reception, and at the same time to reduce the complexity of the present modulation tables.
No patent or utility model whatsoever is known whose features are the object of the present invention.